1. Field of the Invention
The present invention concerns a method for correction of non-reproducible geometry errors of an x-ray C-arm, which occur due to orbital movement of the C-arm during a scan implemented for 3D reconstruction of a patient volume.
2. Description of the Prior Art
An x-ray C-arm apparatus of this type has a base frame on which the C-arm is supported such that it can move orbitally around an isocenter. One end of the C-arm carries an x-ray source and the other end an x-ray receiver, for example a planar detector. Such apparatuses (used in a mobile or stationary manner) are used for, among other things, the 3D reconstruction of a patient volume. 3D exposures are acquired from a number of different angle positions and the patient volume of interest is reconstructed with known calculation methods. The image quality of 3D reconstructions is decisive for its usability, for example for diagnostic purposes. An important parameter for error-free imaging of the 3D world in a 2D image plane is the optimally error-free reproducibility of the position and orientation of the x-ray receiver relative to the isocenter of the C-arm. Due to its own mass and the masses of x-ray source and x-ray receiver, the C-arm twists more or less severely depending on the orbital position. These distortions (that can lie in the centimeter range) are typically compensated in that the movement track of the C-arm is calibrated in an offline method. For this purpose the projection matrices of the C-arm or, respectively, of the x-ray system borne by it are determined using a calibration phantom. Mechanical distortions of the C-arm that occur in at least approximately the same manner given every orbital movement of the C-arm can thereby be compensated. Depending on the type of bearing and drive of the C-arm, a non-reproducible wobbling of the C-arm (ascribed, for example, to tolerance-dependent play) cannot be avoided. A geometry error caused by wobbling leads to a limitation of the spatial resolution to values of approximately 7 to 10 lp/cm for a scan implemented for 3D reconstruction of a patient volume.